A cyclonic eddy sampled by MOCCA floats in the Transkei shelf ecosystem of South Africa

by Dr Tarron Lamont, Department of Environmental Affairs, South Africa

Agulhas Current influence on the Transkei shelf ecosystem of South Africa

The greater Agulhas Current System has been the focus of a substantial amount of research in recent decades due to its key influence on global thermohaline circulation and climate. However, the shelf ecosystems adjacent to the Agulhas Current have received considerably less attention, and have been largely under-sampled, mainly due to the limited societal drivers such as offshore fisheries, mining, port and industrial developments in the region. Under-recognised, is the important role the Transkei shelf ecosystem plays in connecting the semi-sheltered KwaZulu-Natal (KZN) Bight on the east coast of South Africa, with the Agulhas Bank along the south coast. The most well-known example of this is the annual sardine migration from the Agulhas Bank to the KZN Bight, which now supports a growing eco-tourism industry.

As part of a multi-institutional programme, funded by the South African National Research Foundation (NRF), the Institute for Aquatic Biodiversity (SAIAB), the Department of Environmental Affairs (DEA) and ACEP (African Coelacanth Ecosystem Programme), high-resolution, multi-disciplinary sampling has been conducted, on board the RS Algoa, along the Transkei shelf in January/February and July/August 2017 in order to improve the understanding of the shelf oceanography, as well as the response of biological communities to physical forcing associated with the Agulhas Current.

Argo float deployments

During August 2017, a total of six Argo floats, provided by the Euro-Argo ERIC team in the frame of theMOCCA project, were deployed across the Agulhas Current (Figure 1) in order to measure temperature, salinity, and infer flow characteristics of the Current as it flows along the Transkei shelf region. These floats, configured to profile the upper 1000 m on a daily basis, will not only add to and improve the number of Argo float profiles in Western Boundary Current systems, such as the Agulhas Current, but will also provide critical information on the characteristics of the waters forming the offshore boundary of the Transkei shelf ecosystem.

Figure 2. Position of DEA/ACEP Argo floats on 4 September 2017

At the moment, two of the Argo floats (3901912 and 3901914) are moving around a Natal Pulse (cyclonic eddy on the inshore edge of the Agulhas Current), south of Port Elizabeth (Figure 2).